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Jonathan E. Harvey of Utah State University and colleagues present this study of Holocene alluvial deposits along Buckskin Wash, an ephemeral drainage in south-central Utah. After draining from the colorful steps of Utah's Grand Staircase, the stream flows through an arroyo cut approximately 10 meters into a sandy valley fill. Farther downstream, it drains into the world-famous Buckskin Gulch slot canyon, where flood deposits perched more than 10 m above the canyon floor have perplexed scientists for decades. Using detailed stratigraphy, sedimentology, and geochronology, Harvey and colleagues show that there have been at least four cycles of arroyo cutting and filling in the upper watershed since approximately 3,000 years ago, with the most recent arroyo cutting taking place from 1880 A.D. to 1920 A.D. The vast majority of deposits in the slot canyon were emplaced during this event, suggesting that floods are more likely to be preserved when great volumes of sediment are being excavated from upstream valley fills. This important result suggests that chronologies constructed from paleoflood sequences may be controlled more by upstream arroyo cycles than by climatically significant episodes of increased flood frequency or magnitude.

Debate regarding migration of granitic magmas arises from the fact that the link between anatectic source regions, plutons, and batholiths is rarely exposed. The Karakoram shear zone in Ladakh, northwest India, provides a rare opportunity to study magma formation from its origins in partially molten rocks deep in the continental crust, through transfer networks of intrusive sheets that leave the source rocks, to large magmatic bodies in the upper crust. Within the Karakoram shear zone, water influx into rocks at high temperatures triggered extensive melting. Movement along the shear zone squeezed the magma out of the source into a swarm of intrusions into growing fracture planes that crisscrossed shallower and cooler rocks. Henning Reichardt and Roberto F. Weinberg of Monash University use this unique exposure to investigate how the swarm of intrusions provided pathways for removing magma from deep sources and transferring it to the large magmatic bodies now preserved in the Karakoram Mountains. In essence, the complex crisscross pattern is controlled by stress conditions and rock types and formed a multi-kilometric active transfer network across the crust.

***************Cl/Br compositions as indicators of the origin of brines: Hydrogeologic simulations of the Alberta Basin, Canada
Ipsita Gupta et al., Dept. of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA; doi: 10.1130/B30252.1.

The origin and residence time of fluids in the Alberta Basin (Canada) have been debated for more than 40 years, with conflicting conclusions reported by geochemical and hydrogeologic studies. Ipsita Gupta of the University of South Carolina and colleagues use numerical models to determine hydrogeologically feasible scenarios for the origin of these waters in the Alberta Basin over the past 100 million years, using salinity and chlorine/bromine ratios as geochemical constraints. Simulation results suggest that pore fluids in this basin represent a mixture of four end members -- seawater, freshwater, brines formed by evaporation of seawater, and brines derived from salt dissolution -- implying that waters in the basin are likely younger than previously thought. The presence of salt-bearing evaporites in the basin introduces significant uncertainty for interpreting origin of brines in sedimentary basins, but salinity and chlorine/bromine ratios provide valuable new constraints for regional-scale hydrogeological models.

The landscape and seascape of high-latitude areas are to a large degree the result of erosion and deposition by large ice sheets. During successive glacial periods, large ice sheets grew to reach the shelf break and there was a large transfer of sediments eroded from the land and continental shelf areas into the deep sea. Jan Sverre Laberg of the University of Tromso, Norway, and colleagues quantify the glacial erosion in one of the areas affected, the southwestern Barents Sea shelf, and find a glacial erosion rate of 0.4 mm per year averaged over the duration of the ice sheet. They compare their results with published studies of other areas and conclude that this rate is high, indicating that there have been large variations in the rate of glacial erosion affecting the high-latitude areas.

The Slaufrudalur pluton in southeast Iceland is the largest exposed granitic intrusion in the country. It was emplaced with its roof at a depth of 1 km 6 million years ago into flood basalt lavas and later cut by glacial erosion along five valleys. The latter expose sections through the roof and wall contacts of the pluton with the host rock. Steffi Burchardt of the University of Uppsala, Sweden, and colleagues show that these reveal the mechanisms that controlled magma emplacement. In map view, the Slaufrudalur pluton is trapeze-shaped, while in cross sections it exhibits steep walls and a flat roof. Together with the shape, the lack of roof rock disturbance is evidence for magma emplacement by cauldron subsidence; i.e., a piston of rock above an underlying magma chamber was detached along ring faults and subsided into the chamber, while magma rose along the ring fractures and replaced the volume of the sinking piston. The pluton is internally layered, which argues for episodic magma injection and cauldron subsidence. A complex fracture system in the roof rocks is interpreted as a result of repeated inflation and deflation cycles that triggered detachment of blocks of roof material (magmatic stoping).